//paste to position
gw = function(ind){
res = 0;
for( j = ind; j >= 1; j--){
    if((thisComp.layer(j).rotation.value/90)%2 == 0)
        res += thisComp.layer(j).sourceRectAtTime(time).width*thisComp.layer(j).scale[0]/100;
    else
        res += thisComp.layer(j).sourceRectAtTime(time).height*thisComp.layer(j).scale[1]/100;
}
return res
}

i = index-1;
if(!i){
    if((rotation.value/90)%2 != 0){
        [height, width]/2
    }else{
        [width, height]/2
    }
}else{
    x = gw(i) + width/2;
    y = height/2;

    if((rotation.value/90)%2 != 0){
        x = gw(i) + height/2;
        y = width/2;
    }

    [x, y]
}